Electricity energy is widely used on all kinds of occasions. Along with the development of science and technology, people are developing varied power transmission devices to make reasonable use of the electricity energy of power network and meet the demands of different power facilities such as the transformer that changes the voltage, the inverter that implements AC/DC conversion, the transducer that changes the output frequency, various active VAR compensators that keep the electrical source of power network from being polluted, and energy feedback devices. The common drawbacks of these power transmission devices are that their functions are single and the performance could not really achieve an ideal functional mode. For example, the contradiction between the transformation capacity of the transformer and the high temperature insulation and cost of consumables per se; the contradiction between the high voltage transfer of the inverter and the low voltage power switch per se; and the damage brought to the electric machine by the common mode voltage generated during operation wider high frequency as well as the pollution resulted from being transferred to the electrical source of power network. To rectify these defects in performance, the current practice is to adopt corresponding measures to overcome the shortcomings appeared in use, e.g. the above-mentioned inverter, which is mainly formed of rectification portion, filter portion and inversion portion. This type of inverter generates two types of signals, i.e. high frequency and low frequency, during operation. Its voltage waveform is pulse width modulated (PWM) wave, being seriously distorted with a high content of harmonic wave. The high frequency signal generated during operation would form a common mode voltage to enter the electric machine, causing the electric machine to generate heat, resulting in heavy loss, low efficiency and loud noise, thus bringing great damage to the electric machine. Besides, the common mode voltage could be transferred through the ground to the electrical source of power network and cause pollution. To handle said drawbacks caused by the common mode voltage, the current practice is to adopt a transformer to suppress and insulate the common mode voltage, and during the insulation, the voltage imposed on the inversion portion is also reduced, thus ensuring the normal operation of the power switch with low withstand voltage that forms the inversion portion (see Preferred Solutions to Frequency Control of Motor Speed of Medium Voltage Machine released in Electric Transmission, 1st issue, 2002, published by Tianjin Electric Transmission Design Research Institute). The demerit of this measure is the huge bulk of the inverter with heavy weight and high cost, and it could only be used as inverter.